Increasingly, there is a demand for the fabrication of 3-dimensional micron-scale components for micro-electro-mechanical systems (MEMS). Micro-electro-mechanical devices include structures of generally conventional shape and function, e.g., beams, posts levers, wheels, and the like, but of a size that is on the scale of hundreds of microns or smaller. As the general name implies, MEMS often incorporate electro-mechanical elements as sensors and/or actuators including optical components such as electro-mechanical mirrors and the like.
In one approach to fabricating MEMS structural components a 3-dimensional sacrificial resist mold is formed on a substrate for depositing a structural material. Generally, micro-lithographic techniques conventional in micro-integrated circuit fabrication have been used to form shaped structures on substrates. The adaptation of semiconductor manufacturing techniques has also been favored because silicon has been found to be a useful material for making MEMS.
In addition, other structural materials, such as metals, oxides and nitrides have been used for forming MEMS structural components. Generally, the approach includes successive steps of applying a sacrificial resist layer, patterning the resist layer, and forming a structure corresponding to the pattern. The MEMS structures may be formed by either etching a substrate according to the patterned resist layer or by depositing a structural material over the patterned sacrificial resist layer to form a 3-dimensional structure on the substrate surface. Successive stages of patterned deposition and etching may be used to form arrays of larger 3-dimensional MEMS structures.
A particular problem encountered in MEMS manufacture, which is not so often experienced in fabrication of semiconductor devices is the need to provide vertical dimensions and aspect ratios with greater tolerances than those commonly demanded in the fabrication of semiconductor devices. One problem in using sacrificial resists is the tendency of the sacrificial resist to shrink in volume upon curing the resist, including a hard bake process following exposure and development of the resist. As a result, the mass volume of the patterned resist is reduced, altering the critical dimensions of the patterned resist in unpredictable and uncontrollable ways and compromising the critical dimensions of the subsequently formed MEMS structure.
For example, referring to FIG. 1A, is shown a patterned resist layer portion 12 formed over substrate 10. Referring to FIG. 1B, is shown the patterned resist layer portion 12 following a curing process including a hard bake where sidewall portions e.g., 12B are recessed due to resist shrinkage. Referring to FIG. 1C, subsequent deposition of the structural forming layer 14 results in a thinned structural layer e.g., 14B along the sidewalls, resulting in a deformed structural portion compromising design constraints including mechanically weakening the overall structure.
Accordingly, there is a need in the MEMS fabrication art for an improved method to form structural components with improved dimensional accuracy and mechanical integrity including fabricating free-standing structures with high aspect ratios.
It is therefore an object of the invention to provide in the MEMS fabrication art an improved method to form structural components with improved dimensional accuracy and mechanical integrity including fabricating free-standing structures with high aspect ratios, in addition to overcoming other shortcomings of the prior art.